1

Heating systemsHeating systems

2

Source of heatSource of heat

• 99.9% of the cars on the road today use waste heat from the engine cooling system to provide cabin heat.– This heat energy would normally be lost to the radiator.

• Electric heating grids are used on electric vehicles [Nissan Leaf] and plug in electric vehicles [Chevy Volt and Prius Plug-in].– Electric cabin heat uses energy from the battery, reducing

the range the vehicle can travel in electric mode.• Exhaust heat was used for vehicles with air cooled engines

[older Porsche 911 and VW Beetle]

• 99.9% of the cars on the road today use waste heat from the engine cooling system to provide cabin heat.– This heat energy would normally be lost to the radiator.

• Electric heating grids are used on electric vehicles [Nissan Leaf] and plug in electric vehicles [Chevy Volt and Prius Plug-in].– Electric cabin heat uses energy from the battery, reducing

the range the vehicle can travel in electric mode.• Exhaust heat was used for vehicles with air cooled engines

[older Porsche 911 and VW Beetle]

3

Cooling systemCooling system

• The cabin air is heated by the air passing through the heater core.

• The heater core uses hot coolant that would otherwise flow through the radiator.

• The cabin air is heated by the air passing through the heater core.

• The heater core uses hot coolant that would otherwise flow through the radiator.

Heater coreRadiator

Coolant return to water pump

Hot coolant from cylinder head

4

Heater coreHeater core

• The heater core is a heat exchanger.

• Heat from the coolant is transferred to the air passing through the fins.

• The heater core is a heat exchanger.

• Heat from the coolant is transferred to the air passing through the fins.

• The heater core shown here has an aluminum core and plastic side tanks.

• The heater core shown here has an aluminum core and plastic side tanks.

5

Heater coreHeater core

• The heater core is essentially a mini radiator.• Hot coolant passes through thin flat tubes.• Fins attached to the tubes provide additional

surface area for heat to be transferred to the air passing between the tubes.

• The heater core is essentially a mini radiator.• Hot coolant passes through thin flat tubes.• Fins attached to the tubes provide additional

surface area for heat to be transferred to the air passing between the tubes.

Air flow

Coolant flow

Coolant tubes

Fins

6

Brass heater coreBrass heater core

• Brass heater cores are normally found on older vehicles.

• Solder is used to join the components of the brass heater core.

• Unlike aluminum, brass heater cores can be repaired.

• Brass heater cores are normally found on older vehicles.

• Solder is used to join the components of the brass heater core.

• Unlike aluminum, brass heater cores can be repaired.Foam rubber prevents air bypassing the heater core

7

Heater core – side tanksHeater core – side tanks

• Inlet and outlet tubes are located on the heater core’s side tanks.

• If both inlet and outlet tubes are on the same side of the core there must be a baffle in the tank to separate the inlet and outlet sections.

• Inlet and outlet tubes are located on the heater core’s side tanks.

• If both inlet and outlet tubes are on the same side of the core there must be a baffle in the tank to separate the inlet and outlet sections.

Side tank

Inlet

Outlet

Side tank

Baffle

8

Heater hosesHeater hoses

• The heater core is normally located inside a heater box on the cabin side of the firewall.

• The inlet and outlet tubes extend through a hole in the firewall where the heater hoses can be attached.

• The extra connections shown on this van are for a rear heater core.

• The heater core is normally located inside a heater box on the cabin side of the firewall.

• The inlet and outlet tubes extend through a hole in the firewall where the heater hoses can be attached.

• The extra connections shown on this van are for a rear heater core.

Rear heater coolant

lines

Heater core tubes

9

Heater hosesHeater hoses

• Heater hoses are reinforced flexible rubber hoses used to connect the engine to the heater core.

• The engine rotates slightly on the engine mounts as torque is applied to the wheels.

• The heater hoses must flex to accommodate the rotation of the engine.

• Metal pipes are used to conduct coolant in places where there will be no movement – such as lines to rear cabin heater cores.

• Heater hoses are reinforced flexible rubber hoses used to connect the engine to the heater core.

• The engine rotates slightly on the engine mounts as torque is applied to the wheels.

• The heater hoses must flex to accommodate the rotation of the engine.

• Metal pipes are used to conduct coolant in places where there will be no movement – such as lines to rear cabin heater cores.

10

Molded heater hose vs. replacement hoseMolded heater hose vs. replacement hose

• OEM [Original Equipment Manufacturer] hoses a molded to custom fit the application.

• Generic heater hose can be purchased in various inner diameters by the foot from a part store.

• Generic heater hose cannot be bent through sharp angles – it will kink – restricting the flow of coolant if bent through too small a radius.

• Wire wound generic heater hose will not kink.• OEM heater hoses purchased through a dealer is

the best method of repair.

• OEM [Original Equipment Manufacturer] hoses a molded to custom fit the application.

• Generic heater hose can be purchased in various inner diameters by the foot from a part store.

• Generic heater hose cannot be bent through sharp angles – it will kink – restricting the flow of coolant if bent through too small a radius.

• Wire wound generic heater hose will not kink.• OEM heater hoses purchased through a dealer is

the best method of repair.

11

Heater shutoff valveHeater shutoff valve

• The heater shutoff valve blocks the flow of coolant through the heater core when the A/C system is set to A/C max or the temperature control is set to it’s coldest position.

• The shutoff valve can be mounted on the heater core or under the hood

• The heater shutoff valve blocks the flow of coolant through the heater core when the A/C system is set to A/C max or the temperature control is set to it’s coldest position.

• The shutoff valve can be mounted on the heater core or under the hood

12

Heater shutoff valveHeater shutoff valve

• The shutoff valve must have a shaft seal to prevent coolant loss at the point where the shaft enters the valve housing.

• The shutoff valve must have a shaft seal to prevent coolant loss at the point where the shaft enters the valve housing.

• The shutoff valve is operated by a cable or vacuum diaphragm.

• The shutoff valve is operated by a cable or vacuum diaphragm.

13

Vacuum operated heat cutoff valveVacuum operated heat cutoff valve

• When vacuum is applied to the valve shown here the flow of coolant through the heater core is shut off.

• This valve is open whenever there is no vacuum applied [fail safe position]

• When vacuum is applied to the valve shown here the flow of coolant through the heater core is shut off.

• This valve is open whenever there is no vacuum applied [fail safe position]

Vacuum chamber

Vacuum Line

Heater hose

Blower motor resistor

14

Heater fanHeater fan

• Air is circulated through the heater core by an electrically driven ‘squirrel cage’ fan.

• Air enters the center of the fan.

• The rotation of the squirrel cage pushes the air outward by centrifugal force.

• The housing catches the air and sends it on to the heater box assembly.

• Air is circulated through the heater core by an electrically driven ‘squirrel cage’ fan.

• Air enters the center of the fan.

• The rotation of the squirrel cage pushes the air outward by centrifugal force.

• The housing catches the air and sends it on to the heater box assembly.

Centrifugal force pushes the air outward

Air enters through the center of the

squirrel cage

15

Fan motor speed controlFan motor speed control

• A series of wire resistors is used to control the speed of the electric motor.

• Some electronic climate control systems will use a fan control transistor to regulate fan speed.

• A series of wire resistors is used to control the speed of the electric motor.

• Some electronic climate control systems will use a fan control transistor to regulate fan speed.

Medium speed

Base speed

Low speed

Diode

16

Resistor blockResistor block

• The blower motor resistors get very hot when the electric motor is running.

• To keep the resistors cool the resistor block assembly is located in the heater box[heater case] directly downstream from the blower fan.

• The resistor assembly is normally retained by two screws and is usually easy to service.

• Removing the resistor assembly can sometimes allow the technician to get a peak at the evaporator without having to remove the entire heater box assembly.

• The blower motor resistors get very hot when the electric motor is running.

• To keep the resistors cool the resistor block assembly is located in the heater box[heater case] directly downstream from the blower fan.

• The resistor assembly is normally retained by two screws and is usually easy to service.

• Removing the resistor assembly can sometimes allow the technician to get a peak at the evaporator without having to remove the entire heater box assembly.

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Blower motor speed controlBlower motor speed control

• The electric motor that drives the heater fan is called the blower motor.

• The ‘fan motor’ is the electric motor that drives the radiator fan.

• Since 1977 all cars run the blower motor at very low speed whenever the ignition key is on.

• This is intended to prevent injury from carbon monoxide poisoning when the vehicle is not moving but the engine is running to provide heat.

• The electric motor that drives the heater fan is called the blower motor.

• The ‘fan motor’ is the electric motor that drives the radiator fan.

• Since 1977 all cars run the blower motor at very low speed whenever the ignition key is on.

• This is intended to prevent injury from carbon monoxide poisoning when the vehicle is not moving but the engine is running to provide heat.

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Blower motorBlower motor

• The blower motor is typically attached to the heater box by 4 screws and can be easily replaced.

• You can easily test the operation of the motor using jumper wires.

• Most blower motors draw between 10 and 30 amps.

• The blower motor is typically attached to the heater box by 4 screws and can be easily replaced.

• You can easily test the operation of the motor using jumper wires.

• Most blower motors draw between 10 and 30 amps.

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Blower motor circuitBlower motor circuit

The blower motor circuit is made up of:• Fan switch• Ignition switch• Blower motor

The blower motor circuit is made up of:• Fan switch• Ignition switch• Blower motor

• Resistor block• High speed relay• 2 fuses

• Resistor block• High speed relay• 2 fuses

Resistor block

Blower motor

High speed relay

15 amp fuse

Fan switch

30 amp fuse

Ignition switch

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Ignition on – fan switch offIgnition on – fan switch off

• When the ignition switch is turned on power flows through R1 to the motor.

• If R1 is a 10 ohm resistor the motor will receive 1.2 amps of current [about 5% of the motors capacity].

• When the ignition switch is turned on power flows through R1 to the motor.

• If R1 is a 10 ohm resistor the motor will receive 1.2 amps of current [about 5% of the motors capacity].

15 amp fuse

Fan switch

Ignition switch

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Ignition on – fan switch on ‘Low’Ignition on – fan switch on ‘Low’

• When the fan switch is turned to low power flows through R2 and R3 then on to the blower motor.

• If R2 is a 2 ohm resistor and R3 is a 1 ohm resistor the motor will receive 4 amps of electric current. [20%]

• When the fan switch is turned to low power flows through R2 and R3 then on to the blower motor.

• If R2 is a 2 ohm resistor and R3 is a 1 ohm resistor the motor will receive 4 amps of electric current. [20%]

15 amp fuse

Fan switch

Ignition switch

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Ignition on – fan switch on ‘Med’Ignition on – fan switch on ‘Med’

• When the fan switch is turned to medium power flows through R3 then on to the blower motor.

• If R3 is a 1 ohm resistor the motor will receive 6 amps of electric current. [33%]

• When the fan switch is turned to medium power flows through R3 then on to the blower motor.

• If R3 is a 1 ohm resistor the motor will receive 6 amps of electric current. [33%]

15 amp fuse

Fan switch

Ignition switch

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Ignition on – fan switch on ‘High’Ignition on – fan switch on ‘High’

• When the fan switch is turned to high, the fan switch energizes the electromagnet in the high speed relay.

• The high speed relay allows full power to go to the blower motor.

• Blower motors typically draw 20 to 30 amps of current.

• When the fan switch is turned to high, the fan switch energizes the electromagnet in the high speed relay.

• The high speed relay allows full power to go to the blower motor.

• Blower motors typically draw 20 to 30 amps of current.

15 amp fuse

Fan switch

Ignition switch

Note: both fuses are needed for

high speed operation

30 amp fuse

1/4 amp trigger current

20 amp current flow

High speed relay

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Electronic blower motor speed controlElectronic blower motor speed control

• The blower motor speed is electronically controlled by the BCM [Body Control Module] on most late model cars.

• The blower motor speed is electronically controlled by the BCM [Body Control Module] on most late model cars.

BCM

HVAC control head

Fuse box

Blower motor

Blower motor

module [driver]

CAN network

Cabin air temperature

sensor

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Blower motor moduleBlower motor module

• The blower motor module is a high speed electronic switch that turns the power to the electric motor on and off hundreds of times each second.

• This type of electrical control is called a pulse width modulated duty cycle.

• The blower motor module is a high speed electronic switch that turns the power to the electric motor on and off hundreds of times each second.

• This type of electrical control is called a pulse width modulated duty cycle.

Off

On10 mS

10 mS

50% duty cycle1 mS =1/1000 second

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Blower motor moduleBlower motor module

• If the motor is turned on half the time and turned off half the time the motor runs at 50% of it potential speed.

• If we send power to the motor for 1 millisecond and then turn the power off for 19 milliseconds the motor will run at 5 percent of its potential speed.

• If the motor is turned on half the time and turned off half the time the motor runs at 50% of it potential speed.

• If we send power to the motor for 1 millisecond and then turn the power off for 19 milliseconds the motor will run at 5 percent of its potential speed.

Off

On1 mS

19 mS

5% duty cycle1 mS =1/1000 second

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Blower motor moduleBlower motor module

• If we send power to the motor for 19 milliseconds and then turn the power off for 1 millisecond the motor now runs at 95 percent of its potential speed.

• If we send power to the motor for 19 milliseconds and then turn the power off for 1 millisecond the motor now runs at 95 percent of its potential speed.

Off

On

1 mS

19 mS

95% duty cycle1 mS =1/1000 second

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Blower motor control moduleBlower motor control module

• Some of the terminals on the blower motor module send cabin air temperature data back to the BCM

• Some of the terminals on the blower motor module send cabin air temperature data back to the BCM

12 Volt + power feed

12 Volt + power feed

BCM to module communications

29

BCM and HVAC control headBCM and HVAC control head

• When the driver presses the buttons on the HVAC control head a digital signal is sent to the BCM through the vehicle’s CAN network.

• The BCM processes the request and sends the appropriate signals to the heating system actuators.

• When the driver presses the buttons on the HVAC control head a digital signal is sent to the BCM through the vehicle’s CAN network.

• The BCM processes the request and sends the appropriate signals to the heating system actuators.

BCM

HVAC control head

CAN network

Cabin air temperature

sensor

NetworkInterface

chip

30

Electronic blower motor controlElectronic blower motor control

• On initial startup in cold weather the fan is usually turned off so that cold air is not blown onto the passengers.

• If the cabin is cold and defrost mode is selected the fan will normally be commanded to high speed by the BCM.

• On initial startup in cold weather the fan is usually turned off so that cold air is not blown onto the passengers.

• If the cabin is cold and defrost mode is selected the fan will normally be commanded to high speed by the BCM.

• The BCM may run the fan for a few minutes after engine shutdown to dry off the A/C evaporator. This helps prevent the formation of mold on the evaporator core.

• The BCM may run the fan for a few minutes after engine shutdown to dry off the A/C evaporator. This helps prevent the formation of mold on the evaporator core.

Electronic blower motor controller

31

Temperature controlTemperature control

• There are two ways to manually control cabin temperature:– Heater valves regulate the flow of coolant

through the heater core

• Blend doors– Most cars use a blend door system that

maintains a constant flow of hot coolant circulating through the heater core

– Air temperature is controlled by a blend door that mixes heated and unheated air to achieve the desired cabin air temperature

• There are two ways to manually control cabin temperature:– Heater valves regulate the flow of coolant

through the heater core

• Blend doors– Most cars use a blend door system that

maintains a constant flow of hot coolant circulating through the heater core

– Air temperature is controlled by a blend door that mixes heated and unheated air to achieve the desired cabin air temperature

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Heater valveHeater valve

• Heater valves are normally located on the engine compartment side of the firewall where the heater hose meets the heater core inlet.

• The heater valve is normally cable controlled.– Some times an

electronically controlled stepper motor is used

• Heater valves are normally located on the engine compartment side of the firewall where the heater hose meets the heater core inlet.

• The heater valve is normally cable controlled.– Some times an

electronically controlled stepper motor is used

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Heater valvesHeater valves

• The heater valve requires a seal where the valve shaft passes through the valve housing.– This seal is a potential source of coolant leakage

• When the heater valve is located under the hood the cable is subject to corrosion which will cause binding and eventual cable failure.

• The heater valve requires a seal where the valve shaft passes through the valve housing.– This seal is a potential source of coolant leakage

• When the heater valve is located under the hood the cable is subject to corrosion which will cause binding and eventual cable failure.

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Heater boxHeater box

• The heater box is a plastic enclosure that contains the heater core, blower motor and mode doors.

• The heater box is normally located on the cabin side of the firewall.

• Fresh air from the cowl area enters the heater box through a large hole in the firewall.

• The heater box is a plastic enclosure that contains the heater core, blower motor and mode doors.

• The heater box is normally located on the cabin side of the firewall.

• Fresh air from the cowl area enters the heater box through a large hole in the firewall.

Fresh air inlet

Blower motor

Heater core

Defrost outlet

Face level outlet

Foot well outlet

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Blend door – cold positionBlend door – cold position

• The blend door controls the temperature of the air coming out of the air vents at the footwell, face level and defrost vents.

• In the position shown here the air passage to the heater core is totally blocked by the blend door.

• This forces all of the air to bypass the heater core.

• The blend door controls the temperature of the air coming out of the air vents at the footwell, face level and defrost vents.

• In the position shown here the air passage to the heater core is totally blocked by the blend door.

• This forces all of the air to bypass the heater core.

Blend door

Blower motor

Heater core

Defrost outlet

Face level outlet

Footwell outlet

36

Blend door – Hot positionBlend door – Hot position

• In the position shown here the bypass passage is blocked by the blend door.

• This forces all of the air to pass through the heater core.

• In the position shown here the bypass passage is blocked by the blend door.

• This forces all of the air to pass through the heater core.

Blend door

Blower motor

Heater core

Defrost outlet

Face level outlet

Footwell outlet

37

Blend door – mixed positionBlend door – mixed position

• In the position shown here the blend door is in-between the heater core and bypass passage .

• Some of the air goes through the heater core – some bypasses it.

• In the position shown here the blend door is in-between the heater core and bypass passage .

• Some of the air goes through the heater core – some bypasses it.

Blower motor

Heater core

Defrost outlet

Face level outlet

Footwell outlet

38

Mode doors – FootwellMode doors – Footwell

• The footwell mode door diverts air to the defrost/face level duct or foot level outlet.

• Most vehicles have a middle position allowing half of the air to be directed to the face level and haft to go to the footwell vents.

• The footwell mode door diverts air to the defrost/face level duct or foot level outlet.

• Most vehicles have a middle position allowing half of the air to be directed to the face level and haft to go to the footwell vents.

Blower motor

Heater core

Defrost outlet

Face level outlet

Footwell outlet

Footwell mode door

39

Mode doors – DefrostMode doors – Defrost

• When the footwell mode door is closed air is diverted into the defroster/face level duct.

• A second mode door diverts air to either the defrost or face level vents.

• When the footwell mode door is closed air is diverted into the defroster/face level duct.

• A second mode door diverts air to either the defrost or face level vents.

Blower motor

Heater core

Defrost outlet

Face level outlet

Footwell outletFootwell

mode door

Defrost/face level mode door

40

Mode doors – Face levelMode doors – Face level

• The defrost / face level vents diverts air into the face level vents.

• Most cars have individual valves and louvers on the face level vents so that air can be directed differently between the driver and passenger side.

• The defrost / face level vents diverts air into the face level vents.

• Most cars have individual valves and louvers on the face level vents so that air can be directed differently between the driver and passenger side.

Blower motor

Heater core

Defrost outlet

Face level outlet

Footwell outlet

Footwell mode door

Defrost/face level mode door

41

Heater boxHeater box

• To gain access to the heater core and mode doors the entire heater box may need to be removed from the vehicle.

• To gain access to the heater core and mode doors the entire heater box may need to be removed from the vehicle.

Studs attach the heater box to the fire wall

Heater core

42

Recirculating modeRecirculating mode

• Recirculating mode doors are generally not found on vehicles that are not equipped with Air conditioning.

• In recirculating mode the air entering the fan is taken from inside the cabin.

• Recirculating mode doors are generally not found on vehicles that are not equipped with Air conditioning.

• In recirculating mode the air entering the fan is taken from inside the cabin.

Recirculation mode door in

fresh air position

43

Recirculating modeRecirculating mode

• When the outside air temperature is high the air inside the cabin is cooler after the A/C has been running for a few minutes.

• This reduces the amount of work the A/C compressor must do, which helps improve fuel economy while keeping the occupants comfortable.

• When the outside air temperature is high the air inside the cabin is cooler after the A/C has been running for a few minutes.

• This reduces the amount of work the A/C compressor must do, which helps improve fuel economy while keeping the occupants comfortable.

Recirculation mode door in

‘Recirc’ position

44

Blend and mode door actuatorsBlend and mode door actuators

• There are 3 different methods of operating the blend and mod doors.– Cable– Vacuum diaphragm– Electric motor

• Electric motors have become the dominant type of control in the past few years because they allow the vehicle electronic systems a greater degree of control of the HVAC system.

• There are 3 different methods of operating the blend and mod doors.– Cable– Vacuum diaphragm– Electric motor

• Electric motors have become the dominant type of control in the past few years because they allow the vehicle electronic systems a greater degree of control of the HVAC system.

45

Cable actuatorsCable actuators

• Cable actuators work the same as the brake cables on a bicycle.

• Cable actuators work the same as the brake cables on a bicycle.

Outer conduit

Inner cable

Clamp Clamp

Mode door

HVAC control head

Operating lever

46

Cable actuatorsCable actuators

• One disadvantage of a cable is that it’s good at pulling but not so good at pushing.– If the blend door or mode door is binding the

cable may be bent if the operator tries to force it.

• Another disadvantage of cables is that they cannot operate through a sharp angle.

• Cable systems are normally operated by horizontal levers in the control head.

• One disadvantage of a cable is that it’s good at pulling but not so good at pushing.– If the blend door or mode door is binding the

cable may be bent if the operator tries to force it.

• Another disadvantage of cables is that they cannot operate through a sharp angle.

• Cable systems are normally operated by horizontal levers in the control head.

47

Vacuum actuatorsVacuum actuators

• Vacuum actuators – sometimes called vacuum motors were very common in the 1970s and 1980s.

• When no vacuum is applied spring tension pushes the actuating rod outward.

• Vacuum actuators – sometimes called vacuum motors were very common in the 1970s and 1980s.

• When no vacuum is applied spring tension pushes the actuating rod outward.

Vacuum chamber Vacuum

hose

Rubber diaphragm

Actuating rod

Spring

48

Vacuum actuatorsVacuum actuators

• When vacuum is applied to the actuator there is a 10 psi pressure difference operating on the diaphragm.

• If the diaphragm has a surface area of 2 square inches 20 pounds of pulling force will be applied to the actuating rod.

• When vacuum is applied to the actuator there is a 10 psi pressure difference operating on the diaphragm.

• If the diaphragm has a surface area of 2 square inches 20 pounds of pulling force will be applied to the actuating rod.

5 psi intake manifold pressure

Vacuum applied

15 psi atmospheric pressure

49

Dual chamber vacuum actuatorsDual chamber vacuum actuators

• Conventional vacuum actuators are generally either open or closed.

• 3 position vacuum actuators are often used for mode control.

• This allows for an open, ½ open and a closed position.

• Conventional vacuum actuators are generally either open or closed.

• 3 position vacuum actuators are often used for mode control.

• This allows for an open, ½ open and a closed position.

Seal

50

Vacuum controlVacuum control• The vacuum actuators are

controlled by a rotary valve in the HVAC control head.

• The vacuum actuators are controlled by a rotary valve in the HVAC control head.

Vacuum ball

[reservoir]

Check valve

HVAC control head

Rotary valve

Grommet

To intake manifold

• A reserve of vacuum is needed for conditions when there is little or no manifold vacuum [hard acceleration, climbing hills etc].

• A check valve in the vacuum reservoir prevents air from entering the reservoir when vacuum is low

• A reserve of vacuum is needed for conditions when there is little or no manifold vacuum [hard acceleration, climbing hills etc].

• A check valve in the vacuum reservoir prevents air from entering the reservoir when vacuum is low

• Several rotary valves are combined into one assembly to control all the mode door actuators.

• Several rotary valves are combined into one assembly to control all the mode door actuators.

51

Vacuum reservoirVacuum reservoir

• The vacuum reservoir is typically a plastic sphere or canister that is located on the inner fender

• The vacuum reservoir is typically a plastic sphere or canister that is located on the inner fender

52

Vacuum check valveVacuum check valve

• The vacuum check valve closes when the engine is run at high throttle openings. Without it the mode doors would switch to defrost every time you accelerated hard or went up a steep hill

• The vacuum check valve closes when the engine is run at high throttle openings. Without it the mode doors would switch to defrost every time you accelerated hard or went up a steep hill

53

Default no vacuum positionDefault no vacuum position

• When there is no vacuum available to the heater control system the system should be in full defrost mode.– This is the fail safe position

• Most manual heater controls are designed for left-max-defrost.– When all controls are moved to the farthest left

the system has the heat at maximum, the fan speed also at maximum and the mode at full defrost.

• When there is no vacuum available to the heater control system the system should be in full defrost mode.– This is the fail safe position

• Most manual heater controls are designed for left-max-defrost.– When all controls are moved to the farthest left

the system has the heat at maximum, the fan speed also at maximum and the mode at full defrost.

54

Electric actuatorsElectric actuators

• Most modern cars use an electric motor to operate the mode and blend doors.

• There are three types of motors in common use:– Conventional DC electric motors– Stepper motors– Feedback loop control DC motors with a position

sensor

• The BCM controls sends electrical current to the motor when it receives a command from the HVAC control head.

• Most modern cars use an electric motor to operate the mode and blend doors.

• There are three types of motors in common use:– Conventional DC electric motors– Stepper motors– Feedback loop control DC motors with a position

sensor

• The BCM controls sends electrical current to the motor when it receives a command from the HVAC control head.

55

Mode control using conventional DC motorsMode control using conventional DC motors

• Two position mode doors can be controlled using a simple DC electric motor.

• One end of the armature is a worm shaft.• The worm shaft is in mesh with a nylon

sector gear.

• Two position mode doors can be controlled using a simple DC electric motor.

• One end of the armature is a worm shaft.• The worm shaft is in mesh with a nylon

sector gear.

Sector Gear

Mode door shaft

Worm shaft

56

Stepper motorsStepper motors

• A stepper motor normally has four electric terminals.

• When power and ground are applied to 2 terminals the motor will turn a fraction of a turn then stop.

• Power and ground must be applied to a different set of terminals to get the motor to another step.

• The computer can keep track of the position the motor is at by counting the number of times it has changed the polarity of current.

• A stepper motor normally has four electric terminals.

• When power and ground are applied to 2 terminals the motor will turn a fraction of a turn then stop.

• Power and ground must be applied to a different set of terminals to get the motor to another step.

• The computer can keep track of the position the motor is at by counting the number of times it has changed the polarity of current.

57

Feedback motorsFeedback motors

• Feed back motors have a conventional DC electric motor with 2 terminals combined with a position sensor.

• The position sensor is a potentiometer [variable resistor].

• The sensor produces a voltage signal that varies from .5 volts with the door in one position to 4.5 volts as the door moves to the opposite position.

• The BCM converts the voltage into a number [0-to 255] and stores it in RAM memory.

• Feed back motors have a conventional DC electric motor with 2 terminals combined with a position sensor.

• The position sensor is a potentiometer [variable resistor].

• The sensor produces a voltage signal that varies from .5 volts with the door in one position to 4.5 volts as the door moves to the opposite position.

• The BCM converts the voltage into a number [0-to 255] and stores it in RAM memory.

58

Feedback motor controlFeedback motor control

• A 5 volt reference signal plus a ground is applied to the resistor.

• A graphite brush slides back and forth across the resistor as the door opens and closes.

• The voltage at the brush varies from .5 volts to 4.5 volts depending on how far open or closed the door is.

• A 5 volt reference signal plus a ground is applied to the resistor.

• A graphite brush slides back and forth across the resistor as the door opens and closes.

• The voltage at the brush varies from .5 volts to 4.5 volts depending on how far open or closed the door is.

BCM

Resistor strip

ref +5 volt

Ground

Signal

59

Cabin intake airCabin intake air

• Fresh air enters the vehicle through a gap between the trailing edge of the hood and windshield.

• A gasket located between the firewall and underside of the hood prevents engine fumes from entering the HVAC system.

• Fresh air enters the vehicle through a gap between the trailing edge of the hood and windshield.

• A gasket located between the firewall and underside of the hood prevents engine fumes from entering the HVAC system.

Gasket

Screen

Water drain channel

Inlet air plenum

Air inlet

Firewall

60

Cabin air filterCabin air filter

• Many newer cars have an air filter for the HVAC system.• The filter is normally located ahead of the blower motor.

• Many newer cars have an air filter for the HVAC system.• The filter is normally located ahead of the blower motor.

Image courtesy Robert Bosch GMBH

Note: plastic screen has

been removed

Underhood / HVAC gasket